1. Field of the Invention
The present invention relates to a carbon commutator including a carbon layer and a metallic carbon layer, and a method for production thereof.
2. Description of Related Art
Carbon commutators are used in fuel pump motors and the like, and carbon segments come into contact with a brush, the segments being fixed to a riser piece as a metal terminal. Such carbon commutators have a problem in that the metal components contained in the segments are corroded by alcohol, sulfide and the like included in fuel. In this regard, Patent Document 1 (JP 2002-369454A) discloses a carbon commutator in which the segment is composed of two layers, namely, a carbon layer on a surface side and a metallic carbon layer on a riser piece side so as to isolate the metallic carbon layer from alcohol and the like. The metallic carbon layer is provided with protrusions, and the protrusions are press-fitted into holes of the riser piece so as to fix the segments, thereby eliminating the need for soldering and the like. For the metallic carbon layer, instead of copper, brass is used in order to prevent corrosion of metal, and tin is mixed therewith to cause liquid phase sintering. Furthermore, phenol resin is used as a binder in both the carbon layer and the metallic carbon layer.
Patent Document 2 (WO 99/08367) also discloses a carbon commutator including two layers, namely a carbon layer and a metallic carbon layer. The metallic carbon layer is formed by baking at 800 to 850° C. using electrolytic copper powder, tin powder and carbon with phenol resin as a binder. Due to the tin powder being melt, liquid phase sintering occurs, and the carbon in the carbon layer and the metallic carbon layer are sintered with the binder.
The carbon commutators of Patent Documents 1 and 2 use phenol resin as a binder, and therefore baking is performed at a temperature greater than or equal to 700° C. at which the phenol resin is carbonized to function as a binder. However, superior sliding characteristics may be obtained with a carbon layer baked at a lower temperature. The present inventors have found that when a metallic carbon layer using phenol resin as a binder is baked at a low temperature, the metallic carbon layer will have completely insufficient strength. Based on the founding, the present inventors have found a composition for a metallic carbon layer that can be baked at a low temperature and a method for producing a carbon commutator, and the present invention has thus been accomplished.